The present invention relates to mixer circuits and in particular to mixer circuits having a single-ended input and a differential output.
RF mixers are the key blocks of modern radio systems and their parameters determine the main characteristics of the system in which they are used. The most common mixer circuit configurations are those of the Gilbert cell and the Micromixer, shown in FIGS. 1 and 2 respectively.
Each of these mixer circuits receives at its input terminal a single-ended rf input signal and provides at its output a differential signal being the input signal first amplified and subsequently mixed with a signal from a local oscillator. Both of these circuits are easily implemented in IC form and are commonly used in mobile telephones and the like. However, mixers constructed using these circuit configurations exhibit poor noise properties. They also require a supply voltage of 2.7 V or more because each has three transistors in series between supply and ground. This can make them unsuitable for low voltage applications.
Referring to FIG. 1, Gilbert cell circuit 100 receives a single-ended input voltage signal at terminal 130 and a differential local oscillator voltage signal at terminals 140 and 141. Transistors 101, 102, resistors 110, 111 and current source 115 form a differential transconductance amplifier 160 whilst transistors 103-106 form a mixer core 150. An increasing input voltage at terminal 130 will cause an increasing signal current to flow from the collector terminal of transistor 101. Current source 115 and resistors 110, 111 ensure that a complementary decreasing current will flow from the collector electrode of transistor 102. These current signals will be balanced if current source 115 is implemented as a constant current source.
Mixer core 150 receives differential local oscillator signals on terminals 140, 141. When the voltage on terminal 140 is positive, the voltage on terminal 141 will be negative causing transistors 104 and 105 to be switched on and transistors 103 and 106 to be switched off. The collector current of transistor 101 will therefore be routed to output electrode 121 whilst the collector current of transistor 102 will be routed to output terminal 120. The collector currents of transistors 101, 102 will be switched to the opposite output terminal 120, 121 when terminal 141 receives a higher voltage than terminal 140.
The poor noise properties of this mixer configuration are due largely to the thermal noise of resistors 110 and 111 which produce noise directly in the main current paths. Current source 115 will also introduce noise into the output signal, because it experiences quite large voltage swings across its input and output terminals. A significant amount of noise will appear at output terminals 120, 121 as a result of transistors 101 and 102 having their base resistances in series.
The micromixer circuit 200 of FIG. 2 receives a single-ended input signal at input terminal 230 and differential local oscillator signals at terminals 240 and 241. Transistors 201-203 and resistors 210-212 form a transconductance amplifier 260 whilst transistors 204-207 form a mixer core 250.
An increase in voltage at input terminal 230 will cause increased current to flow from the collector electrode of transistor 202 and a decreased current to flow from the collector of transistor 203. The circuit therefore acts as a transconductance amplifier having a single-ended input and a differential output. The output from amplifier 260 is provided on the collector electrodes of transistors 202 and 203, as a differential current signal, to mixer core 250.
Mixer core 250 functions in the same manner as mixer core 150 of the FIG. 1 mixer circuit described above.
Micromixer circuits have very linear characteristics and large dynamic range at radio frequencies but, due to the large number of resistors used in the main current paths, have even worse noise properties than Gilbert cell circuits. There exists a need for a mixer circuit with improved noise properties and low voltage supply requirements.